No .sort(cmp=) in Python 3
No .sort(cmp=) in Python 3

Use functools.cmp_to_key. We can probably(?) be safe and assume people
with python 3 have a recent python 3.

References #71

mrtazz/simplenote.vim,mrtazz/simplenote.vim,sanchon/simplenote.vim,sanchon/simplenote.vim